Fix datarace when UDP/TCP connect fails and we are in nmthread

When we were in nmthread, the isc__nm_async_connect() function executes in the same thread as the isc__nm_connect() and on a failure, it would block indefinitely because the failure branch was setting sock->active to false before the condition around the wait had a chance to skip the WAIT().

This also fixes the zero system test being stuck on FreeBSD 11, so we re-enable the test in the commit.

Closes #2314 (closed), #2332 (closed)